Negative temperature coefficient (NTC) thermistors are often used as the temperature sensing element in electrical temperature control circuits or thermostats. NTC thermistors are highly sensitive, stable, inexpensive to manufacture, and well suited for many temperature sensing applications. For NTC thermistors, a simple exponential model provides accurate results over a limited temperature range, suitable in most applications. The characteristic model of an NTC thermistor is expressed as R=R.sub.o *Exp(Beta/Temperature). Positive temperature coefficient (PTC) thermistors are available but suffer from poor stability and other problems that make them undesirable for many temperature sensing applications.
In a typical electrical temperature control circuit, a NTC thermistor is used as part of a voltage divider to generate a temperature varying voltage signal. The control circuit uses the temperature varying voltage signal to activate or deactivate a relay, which controls the operation of a heater, fan or cooling unit. Often the thermistor is located a considerable distance from the control circuit. Consequently, the lead wires between the thermistor and control circuit are subject to accidental damage and severing. When the connection between the thermistor and control circuit is severed or disconnected, the control circuit can not distinguish between a low temperature condition and a broken or cut lead wire. This severed condition can cause the unregulated control circuit to continuously operate the heater, which may result in collateral damage due to excessive heat or a fire hazard.